1) To test the hypotheses that IV methylnaltrexone can be safely used in reversing methadone-induced gastrointestinal motility changes without withdrawal symptoms in subjects receiving chronic methadone therapy. 2) To test the hypotheses that IV methylnaltrexone can be used in treatment (or prevention) of opioid-induced constipation without interfering with analgesia in chronic pain patients with metastatic malignancy.